本研究では，幼児の自己調整力，見通しをもつ力，好奇心探究心を，協同的問題解決に必要な力と位置づけ，「遊び込む」ことで，これらの力が高まるのかを検討した。また，上越教育大学附属幼稚園における一連の実践研究の分析結果との統合を試みた。そして，質的データと小規模サンプルの量的データとを用いた混合法の意義を論じた。2014 年~2016 年，毎年1 月に，保育者が個々の幼児について質問紙に回答した。対象幼児は，各年， 59，57，47 名だった。縦断的パス解析の結果，「遊び込む」ことで，協同的問題解決に必要な力が高まる可能性が示唆された。これらの要因は，上越教育大学附属幼稚園（2016）で見出された力と重なることも考察された。本研究から以下の3 点が論じられた。（1）量的データからの知見が実践知とマッチすると，数量データと実践知双方の価値を高める。（2）教育研究における混合法は，ある教育介入とその結果との間の複雑な関係を取り扱うことが出来る。（3）個別の学校の小規模データも，個に応じた実践のための貴重な情報となる。The purpose of this article was twofold: 1) to integrate a small-sample quantitative study and field studies as part of the larger ongoing research project at the Kindergarten attached to Joetsu University of Education (K-JUE), and 2) to consider the feasibility or viability of using a mixed method in a single school setting in Japan. This study examined whether toddlers’ cooperative problem-solving abilities are fostered by “totally absorbed play” (Asobi-komu) (K-JUE, 2014, 2015, 2016). Self-regulation, foresight, and curiosity are all viewed as fundamental abilities for cooperative problem-solving. Six kindergarten teachers at K-JUE responded to a questionnaire about eight abilities of the children in their classrooms and the children’s “degree of being absorbed in play” in January 2014, 2015, and 2016 (N = 59, 57, 47, respectively). The results of longitudinal path analyses showed that the “totally absorbed play” enhanced children’s fundamental abilities in cooperative problem-solving. These findings were consistent with those found in case studies. Finally, the significance of this mixed-method study using mixed methods include the following: a) This study confirms that fitting quantitative data with teachers’ practical knowledge enriches the practical and contextual value of the quantitative data. Teachers’ awareness of the consistency between their practical knowledge and quantitative data (although derived from a small sample) gave them confidence in their practices. b) Mixed methods were able to accommodate the complex relations between some educational interventions, children’s psychological states, and performances. c) Even with a small sample, using quantitative data on a single school can be of great practical value in promoting children’s development over the span of several years.